Phone shutting off by itself suddenly?

view story

http://androidforums.com – So i recently went back to stock unrooted using the FG32 1 click method and everything went seemingly fine. About a day later though something weird happened, when I wouldn't be using my phone constantly it would seem to turn off by itself and i would have to hold down to power button for a long time for it to just turn back on. As long as im using it though it is fine, but if I leave it alone for any longer than 5-10 minutes it will do this. Any possible reason for this? or way I could troubleshoot it? (General)